Fossil fuels supply the world with not only energy but also important feedstocks for chemical industry. However, the shrinking availability of fossil reserves and the deteriorating environment compel people to explore renewable alternatives for the production of fuels, chemicals, and pharmaceuticals. Fortunately, the metabolic diversity of biological systems provides us with an extremely rich chemical repertoire. In recent years, the development of metabolic engineering has enabled the establishment of microbial chemical factories by constituting heterologous or non-natural biosynthetic pathways into genetically advantageous microbial hosts (Ajikumar et al., 2010 Science 330:70-4; Anthony et al., 2009 Metab. Eng. 11:13-9; Atsumi et al., 2008 Metab. Eng. 10:305-11; Huang et al., 2013 Biotechnol. Bioeng. 110:3188-96; Lin et al., 2013b Metab. Eng. 18:69-77; Lin et al., 2012 Microb. Cell Fact. 11:42; Shen et al., 2008 Metab. Eng. 10:312-20; Shen et al., 2012 J. Ind. Microbiol. Biotechnol. 39:1725-9; Zhang et al., 2008 Proc. Natl. Acad. Sci. U.S.A. 105:20653-8).
Muconic acid (MA) is a platform chemical that serves as the precursor to several bio-plastics. It is also a naturally-occurring metabolite. Muconic acid is present in biological systems as an intermediate in the microbial degradation of aromatic hydrocarbons (Fuchs et al., 2011 Nat. Rev. Microbiol. 9:803-16). In past 20 years, many efforts have been made for the microbial production of muconic acid. Draths and Frost reported the earliest study on the artificial biosynthesis of muconic acid in Escherichia coli from renewable carbon source glucose (Draths et al., 1994 J. Am. Chem. Soc. 116:399-400). By introducing three heterologous enzymes 3-dehydroshikimate dehydratase, protocatechuic acid decarboxylase and catechol 1,2-dioxygenase (CDO), the carbon flux was redirected from the E. coli native shikimate pathway to the biosynthesis of muconic acid. Metabolically optimized strains carrying this artificial pathway were able to produce up to 2.4 g/L of muconic acid via two-stage bioconversion in shake flasks (Draths et al., 1994 J. Am. Chem. Soc. 116:399-400) and 38.6 g/L via fed-batch fermentation (Niu et al., 2002 Biotechnol. Prog. 18:201-11). Afterwards, the same pathway was reconstituted in Saccharomyces cerevisiae (Weber et al., 2012 Appl. Environ. Microbiol. 78:8421-30), and the highest titer reported was nearly 141 mg/L (Curran et al., 2013 Metab. Eng. 15:55-66).
Muconic acid is easily converted into adipic acid by chemical hydrogenation, and adipic acid is a direct building block for nylon-6,6 and polyurethane (Sun et al., 2013 Appl. Environ. Microbiol. 79:4024-30). In addition, muconic acid is a synthetic precursor to terephthalic acid, a chemical used for manufacturing polyethylene terephthalate (PET) and polyester (Curran et al., 2013 Metab. Eng. 15:55-66). The global production of adipic acid and terephthalic acid is 2.8 and 71 million metric tons, respectively (Curran et al., 2013 Metab. Eng. 15:55-66).
Salicylic acid (SA) is an important drug precursor mainly used for producing pharmaceuticals such as aspirin and lamivudine (an anti-HIV drug). Like muconic acid, it is a naturally-occurring metabolite. In biological systems, salicylic acid serves not only as a plant hormone (Chen et al., 2009 Plant Signal Behav. 4:493-6) but also as a biosynthetic precursor of bacterial siderophore (Gaille et al., 2002 J. Biol. Chem. 277:21768-75). Salicylic acid esters and salts used in sunscreens and medicaments account for another large portion of salicylic acid consumption. The global market for salicylic acid products was estimated to be $292.5 million in 2012 and is expected to reach $521.2 million in 2019, growing at an annual increase of 8.6% (“Salicylic Acid Market for Pharmaceutical, Skin care, Hair care and Other Applications-Global Industry Analysis, Size, Share, Growth, Trends, and Forecast 2013-2019,” 2013 Transparency Market Research).
Muconic acid and salicylic acid are thus naturally-occurring organic acids having great commercial value. Muconic acid is a potential platform chemical for the manufacture of several widely-used consumer plastics; while salicylic acid is mainly used for producing pharmaceuticals, skincare and haircare products. At present, commercial production of muconic acid, salicylic acid, adipic acid, and terephthalic acid predominantly relies on organic chemical synthesis using petroleum-derived chemicals, such as benzene, as starting materials. These chemical synthesis processes are considered nonrenewable and environmentally unfriendly. Therefore, it is of great importance to develop “green” synthetic approaches that can utilize renewable feedstocks.